MOSCOW — Saboteurs blew up the main power lines leading into Crimea early Sunday, plunging the disputed peninsula into darkness and prompting the Russian government to impose a state of emergency there.

Most of the more than 1.8 million residents of the peninsula, annexed by Russia last year, lacked electricity, Russian news agencies reported, although backup generators were being used to provide power to hospitals and for other vital purposes.

Sergei V. Aksyonov, the prime minister of Crimea, asked people not to use electric appliances like heaters — the temperature was more than 60 degrees — and said streetlights would remain off to preserve power for more essential purposes.

The government imposed a 10 p.m. curfew on restaurants and entertainment sites.

The four trunk lines running from Ukraine to Crimea were all damaged, and two districts in the Ukrainian region of Kherson were also without electricity, Volodymyr Demchyshyn, the energy minister of Ukraine, said in a statement. Most of the electricity used in Crimea still comes from Ukraine even after Russia annexed it in March 2014.

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A Crimean Tatars flag was wrapped around broken power lines near the village of Chaplynka on Sunday.CreditHromadske Tv/European Pressphoto Agency

Unknown people presumed to be Ukrainian nationalists knocked out all four of the main electricity lines running through the Kherson region — the first two were severely damaged on Friday and two more by explosions just after midnight on Sunday morning.

Officials in Ukraine said some power could be restored in 24 hours and all four destroyed pylons restored within two or three days, but they said unidentified demonstrators were preventing repair crews from getting access to the site.

Six repair brigades had arrived in the area and were ready to start work, Yuri Kasich, of Ukrenergo, the state agency that runs the power grid, was quoted as saying by Ukrainian news services. Mr. Kasich declined to say who was causing the problem, but he said workers had yet to start the repair work.

The government in Crimea imposed rolling blackouts on most residential neighborhoods and announced that it had enough fuel on hand for emergency generators to keep them running for a month.

Crimea announced a day off for nongovernment workers on Monday, and shut down public services that use a lot of electricity, like the trolley-bus service in the port city of Sevastopol, replacing it with regular buses on some routes.

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An employee at an electronics store in Simferopol, Crimea, used a lamp to show items to customers.CreditAlexander Polegenko/Associated Press

Russia’s Black Sea fleet, which has its headquarters at Sevastopol, announced that it had not been affected by the power failure.

Reports in Ukraine said that the police had clashed with activists from the right-wing nationalist Right Sector movement in the area on Saturday after the initial damage. Ukraine still claims the peninsula.

When the first two lines were shut down on Friday, Crimea’s Ministry of Fuel and Energy warned residents that disruptions in the power supply and other services like cellphones were possible and suggested that they stock up on batteries, water and other essentials.

Russia plans to replace the electricity supply from Ukraine with power lines from the Russian mainland, but those are not yet complete.

In September, activists from the Tatar minority, which accuses Russia of repressing its members in Crimea, tried to blockade the roads in the Kherson region used to transport food and other goods to Crimea.

Russia’s annexation of Crimea and its support for separatists in eastern Ukraine provoked the worst crisis in relations between Russia and the West since the Soviet Union collapsed in 1991, with the West imposing economic sanctions.

Correction:

An earlier version of this article misspelled the name of the city that is a Russian naval port. It is Sevastopol, not Sebastopol.

Alisa Sopova contributed reporting from Kiev, Ukraine.

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A version of this article appears in print on , on Page A3 of the New York edition with the headline: Crimea in Dark After Power Lines Are Blown Up . Order Reprints | Today’s Paper | Subscribe